i) Field of the Invention
This invention relates to an exchange mass for recovery of a value metal from solution by reaction with a replacement metal; to a device housing such an exchange mass for such recovery; and to a method of such recovery.
ii) Description of Prior Art
It is desirable to recover value metals, for example, precious metals from otherwise waste or spent solutions containing them; by way of example, reference may be made to the recovery of silver from spent photographic solutions such as fixer solutions.
Silver recovery from used photoprocessing solutions by metallic replacement is carried out by using chemical recovery cartridges or metal replacement cartridges. Silver recovery by metal replacement is based on the fact that different chemical elements have different affinities for electrons. If a metal is in an elemental form and it has lower affinity for electrons, it will react with metals that are in ionic forms that have higher affinity for electrons. Iron has a lower affinity for electrons than silver, so upon contacting with photo processing solutions containing silver ions, iron metal will react with the silver ions. Iron metal becomes iron ions and enters solution while silver ions originally in the solution becomes silver metal. Iron is above silver in the electromotive force series. Some other metals will also work, such as aluminum and zinc. Iron in various forms is the most widely used because it is less expensive and more environmentally friendly.
Thus silver ion (in a complex form) is chemically reduced to silver metal by iron metal, and iron metal is chemically oxidized to iron ions in a metal replacement process.
Suitably the iron is supported in an elongate cartridge such as described by Woog in U.S. Pat. Nos. 4,662,613 and 4,325,732.
Different problems are encountered when metal replacement cartridges are used. The biggest problem is low efficiency in terms of iron usage. Theoretically, one gram of iron could recover 3.86 grams of silver (Fe, atomic weight, 55.847; Ag, atomic weight, 107.868; chemical reaction, Fe+2Ag(I)=Fe(II)+2Ag). In reality, most commercial cartridges have an efficiency of recovering 0.1 to 0.5 grams of silver per gram of iron in the cartridge. Part of the iron is consumed by the acid in the fixer. The more acidic the fixer, the lower the efficiency would be. However, the major cause of low efficiency is the so-called channeling phenomenon.
Channeling in a metal replacement cartridge occurs because of two reasons. Steel wool fibres tend to cluster together. Most manufacturing processes are not able to eliminate gaps between steel wool clusters or steel wool layers. Another reason is that silver ions and acids in the fixer react and consume steel wool, thus enlarging the fixer flow paths. Fixer will preferably flow through these gaps and paths so that the silver ions have less chance of reacting with steel wool, leading to high silver concentrations in the cartridge effluent. It is likely that a large portion of the initial steel wool in the MRC still remains when breakthrough point is reached.